In 1992, the Democratic Party faced a challenge on the issue of abortion. Pennsylvania’s governor, Robert Casey, a Democrat dedicated to the working class, asked to speak at the national convention in New York City. He wanted to propose a pro-life plank for the party platform, mostly as a way of affirming his Catholic beliefs.

He fully understood that the motion would be voted down, but the Democratic Party refused to permit him even to air his views, so great was his heresy. “That sent a strong signal to working-class Catholic and evangelical voters that if they did not fall into line on this one issue they were no longer welcome in the party,” writes Mark Lilla in “The Once and Future Liberal,” his brief but brilliant book that comes out later this month.

I wonder if today the Democrats are making the same mistake on immigration. To be clear, I think the bill that the Republicans rolled out this week is bad public policy and mean-spirited symbolism. But that’s beside the point. Lilla acknowledges that he is a pro-choice absolutist on abortion, but he argues that a national party must build a big tent that accommodates people who dissent from the main party line on a few issues.

In Lilla’s view, there is a larger crisis within American liberalism. When he visited the online home page of the Republican National Committee, he found a statement of broad principles that guide the party, starting with the Constitution and ending with immigration. On the Democrats’ website, by contrast, he noticed a set of links to “People,” and when he clicked on them he got to pages specifically designed to appeal to one group or another — women, Hispanics, Native Americans, African Americans, Asian Americans. Alluding to Lebanon’s system of power-sharing among religious and ethnic groups, Lilla writes, “You might think that, by some mistake, you have landed on the website of the Lebanese government — not that of a party with a vision for America’s future.” (The Democratic National Committee’s home page now features the party’s platform more prominently.)

There have been two different agendas for American liberalism, according to Lilla. The first was Franklin D. Roosevelt’s — a collective, national effort to help all Americans participate in the country’s economic and political life. Its symbol was two hands shaking, an affirmation of the binding strength of national unity. The more recent liberal project has been centered on identity, affirming not unity but difference, nurturing and celebrating not national identities but sub-national ones. “A recurring image of identity liberalism is that of a prism,” Lilla notes, “refracting a single beam of light into its constituent colors, producing a rainbow. This says it all.”

(The Washington Post)

Immigration is the perfect issue on which Democrats could demonstrate that they care about national unity and identity — and that they understand the voters for whom this is a core concern. Look at the Democracy Fund’s voter study done in the wake of the 2016 election. If you compare two groups of voters — those who voted for Barack Obama in 2012 and Hillary Clinton in 2016, and those who voted for Obama in 2012 and Donald Trump in 2016 — the single biggest divergence on policy is immigration. In other words, there are many Americans who are otherwise sympathetic to Democratic ideas but on a few key issues — principally immigration — think the party is out of touch.

And they are right. Consider the facts. Legal immigration in the United States has expanded dramatically over the last five decades. In 1970, 4.7 percent of the U.S. population was foreign-born. Today, it’s 13.4 percent. That’s a large shift, and it’s natural that it has caused some anxiety.

The anxiety is about more than jobs. In his 2004 book “Who Are We?,” Harvard University scholar Samuel Huntington pointed out that the scale, speed and concentration of Mexican migration into America after 1965 were without precedent in the country’s history and could provoke a backlash.

He asserted that America had more than just a founding ideology; it had a culture that had shaped it powerfully. “Would America be the America it is today if in the seventeenth and eighteenth centuries it had been settled not by British Protestants but by French, Spanish, or Portuguese Catholics?” Huntington asked. “The answer is no. It would not be America; it would be Quebec, Mexico, or Brazil.” He advocated some modest limits on immigration and, more important, a greater emphasis on assimilation.

Democrats should find a middle path on immigration. They can battle President Trump’s drastic solutions but still speak in the language of national unity and identity. The country’s motto, after all, is “out of many, one” — not the other way around.